1. Field of the Invention
The invention relates to a nonaqueous electrolyte secondary battery.
2. Description of Related Art
Compared to existing batteries, nonaqueous electrolyte secondary batteries such as lithium ion secondary batteries (lithium secondary batteries) are lighter in weight and have higher energy density. Therefore, in recent years, the nonaqueous electrolyte secondary batteries have been used as so-called portable power supplies in personal computers, mobile terminals, and the like or as power supplies for driving vehicles. Particularly, the lithium ion secondary batteries which are light-weight and bring about high energy density are preferably used as high-output power supplies for driving vehicles such as an electric vehicle (EV), a hybrid vehicle (HV), and a plug-in hybrid vehicle (PHV).
Typically, a nonaqueous electrolyte secondary battery includes a positive electrode having a positive electrode active material layer, a negative electrode having a negative electrode active material layer, and a nonaqueous electrolyte. It is a battery which is charged and discharged by the travel of charge carriers (for example, lithium ions) in the electrolyte between the electrodes. At the time of charging the nonaqueous electrolyte secondary battery, charge carriers (typically, lithium ions) are released (dissociated) from the positive electrode active material configuring the positive electrode active material layer and are absorbed (inserted) into the negative electrode active material configuring the negative electrode active material layer. Inversely, at the time of discharging the nonaqueous electrolyte secondary battery, charge carriers (typically, lithium ions) are released (dissociated) from the negative electrode active material and are absorbed (inserted) into the positive electrode active material. As the charge carriers (typically, lithium ions) are absorbed into and released from the active materials along with the charge and discharge of the nonaqueous electrolyte secondary battery as described above, the positive and negative electrode active materials (that is, positive and negative electrode active material layers having the active materials) expand and contract.
Typically, such a nonaqueous electrolyte secondary battery is established by accommodating an electrode unit, which consists of a positive electrode and a negative electrode laminated to each other together with a separator interposed therebetween, and an electrolyte in some cases. As the structure of the electrode unit, a laminate-type electrode unit in the form of a laminate of a plurality of planar electrode units, a roll-type electrode unit obtained by winding up a long sheet-like electrode unit in the form of a scroll, or the like is known. If the electrode unit is configured as above, a reaction area between the positive and negative electrodes can be enlarged, and thus the energy density and the output can be improved. Herein, as the separator, a porous film made of a resin is typically used. Such a separator functions to electrically insulate the positive electrode from the negative electrode and to hold the nonaqueous electrolyte. Examples of technical documents relating to such a nonaqueous electrolyte secondary battery include Japanese Patent Application Publication No. 2008-078008 and Japanese Patent Application Publication No. 2012-074403.
In a case where the nonaqueous electrolyte secondary battery configured as above is used in the application in which high-rate charge and discharge are repeated (for example, in a case where the battery is mounted on a vehicle), due to the expansion and contraction of the positive and negative electrode active materials (positive and negative electrode active material layers) accompanied by the charge and discharge, the separator may be pressed, thus pores of the separator may be crushed, and hence the nonaqueous electrolyte held in the pores may be extruded from the electrode unit. As a result, variation occurs in the amount of the nonaqueous electrolyte held in the electrode unit, and accordingly, in the electrode unit, a portion holds a large amount of nonaqueous electrolyte while the other portion holds a small amount of (lack of) nonaqueous electrolyte in some cases. In a case where the roll-type electrode unit is used as an electrode unit, the amount of the held nonaqueous electrolyte varies between the end portion and the central portion in the direction of the winding axis in some cases (that is, variation occurs in the amount of the nonaqueous electrolyte in some cases). Within the electrode unit, in the portion having a small amount of (lack of) nonaqueous electrolyte, so-called liquid shortage easily occurs. In the portion having a small amount of nonaqueous electrolyte (typically, the portion where the liquid shortage occurs), the amount of the nonaqueous electrolyte present is less than the necessary quantity, and the overall charge and discharge performance of the battery tends to deteriorate. Furthermore, within the electrode unit, a battery reaction mainly occurs in the portion having a relatively large amount of nonaqueous electrolyte, and thus the deterioration of such a portion tends to be accelerated. None of the aforementioned phenomena are preferable because they become a factor of performance deterioration (increase of battery resistance, capacity deterioration, and the like). Especially, for the nonaqueous electrolyte secondary battery used in the application in which a high level of high-rate charge and discharge characteristics is required, it is important to suppress the performance deterioration resulting from the variation in the amount of the nonaqueous electrolyte in the electrode unit.